Method of fabrication of an array of graded refractive index microlenses integrated in an image sensor

ABSTRACT

Methods and devices that incorporate microlens arrays are disclosed. An image sensor includes a pixel layer and a dielectric layer. The pixel layer has a photodetector portion configured to convert light absorbed by the pixel layer into an electrical signal. The dielectric layer is formed on a surface of the pixel layer. The dielectric layer has a refractive index that varies along a length of the dielectric layer. A method for fabricating an image sensor includes forming an array of microlenses on a surface of the dielectric layer, emitting ions through the array of microlenses to implant the ions in the dielectric layer, and removing the array of microlenses from the surface of the dielectric layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of patent application Ser. No.13/306,094, filed Nov. 29, 2011, which claims the benefit of provisionalpatent application No. 61/489,393, filed May 24, 2011, which are herebyincorporated by reference herein in their entireties. This applicationclaims the benefit of and claims priority to patent application Ser. No.13/306,094, filed Nov. 29, 2011, and provisional patent application No.61/489,393, filed May 24, 2011.

FIELD OF THE INVENTION

The present invention relates generally to imaging devices, and moreparticularly, to imaging devices incorporating microlens arrays.

BACKGROUND OF THE INVENTION

Image sensors convert optical light to an electrical signal.Conventional image sensors are used predominantly in digital cameras,and may fall into one of two categories: charge-coupled device (CCD)image sensors and complementary metal-oxide-semiconductor (CMOS) imagesensors.

Image sensors are formed from an array of pixels, each of which isconverts received light into an electrical signal. Image sensors mayinclude arrays of microlenses positioned above the pixels in order tofocus the light received by the pixel. The effectiveness of a pixel atconverting received light into an electrical signal is dependent atleast in part on the amount of light received in the photodetectorregion of the pixel. Thus, improvements are desired that focus lighttoward the photodetector region of a pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings, with likeelements having the same reference numerals. When a plurality of similarelements are present, a single reference numeral may be assigned to theplurality of similar elements with a small letter designation referringto specific elements. When referring to the elements collectively or toa non-specific one or more of the elements, the small letter designationmay be dropped. According to common practice, the various features ofthe drawings are not drawn to scale unless otherwise indicated. To thecontrary, the dimensions of the various features may be expanded orreduced for clarity. Included in the drawings are the following figures:

FIG. 1 is a cross-sectional diagram illustrating an example image sensorin accordance with aspects of the present invention;

FIG. 2 is a chart illustrating an example refractive index for adielectric layer of the image sensor of FIG. 1;

FIG. 3 is a flowchart illustrating an example method for fabricating animage sensor in accordance with aspects of the present invention;

FIG. 4 is a diagram illustrating an example forming step of the methodof FIG. 3;

FIG. 5 is a diagram illustrating an example emitting step of the methodof FIG. 3; and

FIG. 6 is a diagram illustrating an example removing step of the methodof FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The image sensors described herein are usable for a variety ofelectronic devices including, for example, digital cameras. Thedisclosed image sensors and methods achieve improvements in focusinglight received by the image sensor pixels.

The example embodiments disclosed herein are particularly suitable foruse in conjunction with complementary metal-oxide-semiconductor (CMOS)image sensors. Nonetheless, while the example embodiments of the presentinvention are described herein in the context of CMOS image sensors, itwill be understood by one of ordinary skill in the art that theinvention is not so limited.

Referring now to the drawings, FIG. 1 illustrates an example imagesensor 100 in accordance with aspects of the present invention. Imagesensor 100 may be part of an electronic device such as, for example, adigital camera. As a general overview, image sensor 100 includes a pixellayer 110 and a dielectric layer 120. Additional details of image sensor100 are described below.

Pixel layer 110 converts light received by the image sensor 100 into anelectric signal. In particular, pixel layer 110 includes photodetectorportions 115 that are configured to convert light absorbed by pixellayer 110 into the electrical signal. Briefly, a semiconductor p-njunction diode is often used for the detection of light signals. The p-njunction is typically reverse biased, creating a depletion region in avolume surrounding the p-n junction. As such, light illuminating the p-njunction cause electrons in the valance band of the semiconductormaterial to transition into the conduction band, generatinghole-electron pairs in the depletion region which are swept out of thedepletion region in opposite directions. A change in junction potentialdue to collapse of the depletion region is detected as the signalindicative of the intensity of the light absorbed by pixel layer 110.

A p-n junction diode intended for use as a photodetector is oftenreferred to as a photodiode. Various physical mechanisms act to limitthe ability of the photodiode and photodiode arrays to detect andspecially resolve low levels of light. Important among these mechanismsare noise, surface reflectivity, leakage currents, and cross-talk. Noisemay be due to random fluctuations in light signal intensity, thermalmechanisms, and other causes. Other characteristics of the photodiode,such as depth of the junction below the semiconductor surface and widthof depletion region, also influence the sensitivity of the photodiode tothe incident light.

Dielectric layer 120 is formed on a surface of pixel layer 110.Dielectric layer 120 may be formed on the front surface or the backsurface of pixel layer 110 relative to a source of the light absorbed byrespective photodetector portions 115 of pixel layer 110. In an exampleembodiment, dielectric layer 120 is a silicon nitride passivation layerdeposited on the surface of pixel layer 110. However, it will beunderstood by one of ordinary skill in the art from the descriptionherein that dielectric layer 120 may be any layer of material formed ona surface of pixel layer 110 during fabrication of image sensor 100.

Suitable processes for the fabrication of pixel layer 110 and the basematerial of dielectric layer 120 are described, for example, in U.S.Patent Application Publication No. 2010/01640428 to Manabe, the contentsof which are incorporated herein by reference for their teaching on thefabrication of image sensors and pixels. Additional processes forcompleting the fabrication of dielectric layer 120 are described ingreater detail below.

Dielectric layer 120 performs the function of a microlens array in imagesensor 100. Dielectric layer 120 has a refractive index that variesalong a length of dielectric layer 120. The refractive index ofdielectric layer 120 may vary periodically (e.g., like a sine wave) asone follows dielectric layer 120 from pixel to pixel across a surface ofpixel layer 110. Desirably, the refractive index of dielectric layer 120reaches a relative maximum value at areas centered above the respectivephotodetector portions 115 in pixel layer 110, in order to focus thelight received by image sensor 100 on the photodetector portions 115.

In an example embodiment, dielectric layer 120 includes regions ofimplanted ions 160 in a base material of the dielectric layer 120. Theimplanted ion regions 160 cause the variance in the refractive index ofdielectric layer 120 by changing the optical properties of the basematerial of the dielectric layer 120. The pattern of the implanted ionregions 160 is dependent on the positions of the photodetector portions115 in pixel layer 110, and on the materials of both the implanted ionsand the dielectric layer 120, as set forth below.

For one example, the implanted ions may increase the refractive index ofthe base material. As shown in FIG. 1, the implanted ion regions 160 arecentered above respective photodetector portions 115 in pixel layer 110(i.e. between points A, B, and C in FIG. 1), in order to focus the lightreceived by image sensor 100 on the photodetector portions. As shown inFIG. 2, the implanted ion regions 160 increase the refractive index ofdielectric layer 120 in the areas above respective photodetectorportions 115 (i.e. between corresponding points A, B, and C in FIG. 2).

For another example, the implanted ions may decrease the refractiveindex of the base material. The implanted ions in this example maycomprise argon, and the base material may comprise silicon nitride. Inthis example, the implanted ion regions 160 may be centered at theboundaries or edges between respective photodetector portions 115 inpixel layer 110, in order to focus the light received by image sensor100 on the photodetector portions.

It will be understood that the above embodiments are illustrative ofsuitable materials for use in dielectric layer 120, and are not intendedto be limiting. Other suitable base materials include, for example,silicon oxide, silicon carbide, or SiCN. Other suitable ions forimplantation in the base material include, for example, nitrogen,fluorine, or germanium.

FIG. 3 is a flowchart illustrating an example method 200 for fabricatingan image sensor in accordance with aspects of the present invention. Theimage sensor includes a pixel layer and a dielectric layer formed on thepixel layer. Method 200 may desirably be implemented, for example, toform a CMOS image sensor for a digital camera. As a general overview,method 200 includes forming an array of microlenses, emitting ionsthrough the array, and removing the array. Additional details of method200 are described herein with respect to the components of image sensor100.

In step 210, an array of microlenses is formed. In an exampleembodiment, an array of microlenses 150 is formed on a surface ofdielectric layer 120, as shown in FIG. 4. Microlens array 150 maycomprise microlenses formed from a polymer material such as conventionalphotoresist material used for microlens fabrication. Microlens array 150may be formed by conventional photolithographic processes, as would beunderstood to one of ordinary skill in the art. Briefly, a heat-curablepolymer is coated on the surface of the dielectric layer in liquid form.The polymer forms the lens shapes by means of the thermal process thatcauses the modification of the surface profile. The lens shapes are thenset through heat curing process.

In step 220, ions are emitted through the array of microlenses. In anexample embodiment, ions are emitted through the array of microlenses150 to form implanted ion regions 160 in dielectric layer 120, as shownin FIG. 5. The ions may be, for example, argon ions. The ions maydesirably be emitted in a direction normal to the surface of dielectriclayer 120. However, ions may alternatively be emitted at an obliqueangle relative to the surface of dielectric layer 120. The ions areemitted with the proper energy and density to penetrate through themicrolens array 150 and into dielectric layer 120. For example, the ionsmay be emitted with an energy of approximately 30 keV. The microlensarray 150 impedes the ions such that fewer ions are implanted beneaththe thicker regions of microlens array 150 and more ions are implantedbeneath the thinner regions of microlens array 150.

The formation of microlens array 150 changes the reflective profile ofimage sensor 100. Thus, during step 220, microlens array 150 acts as ascreen for the implantation of ions in dielectric layer 120. The degreeto which ions are implanted in dielectric layer 120 varies based on thepositioning and thickness of microlens array 150.

In one embodiment, microlens array 150 is formed in step 210 such thatthe points of minimum thickness of the array are centered aboverespective photodetector portions of pixel layer 110. In thisembodiment, areas of dielectric layer 120 above respective photodetectorportions would experience the most ion implantation during step 220. Inanother embodiment, microlens array 150 is formed in step 210 such thatthe points of maximum thickness of the array are centered aboverespective photodetector portions of pixel layer 110. In thisembodiment, areas of dielectric layer 120 above respective photodetectorportions would experience the least ion implantation during step 220. Inthis way, the positioning and concentration of implanted ion regions 160in dielectric layer 120 may be controlled by the positioning andthickness of microlens array 150 formed during step 210.

In step 230, the array of microlenses is removed. In an exampleembodiment, microlens array 150 is removed from the surface ofdielectric layer 120, as shown in FIG. 6. Microlens array 150 may beremoved by conventional photolithographic processes such as wet cleaningor ashing, as would be understood to one of ordinary skill in the art.

The resulting dielectric layer 120 has implanted ion regions 160 in apattern corresponding to that of a microlens array. The implantation ofions in the base material of dielectric layer 120 physically damages thematerial, thereby changing its refractive index. Because the implantedion regions 160 change the refractive index of the base material ofdielectric layer 120, the dielectric layer 120 of the present inventionperforms the function of a microlens array in image sensor 100.

Aspects of the present invention relate to methods and devices thatincorporate microlens arrays.

According to one aspect of the present invention, a method forfabricating an image sensor is disclosed. The image sensor has a pixellayer and a dielectric layer. the method comprising the steps of formingan array of microlenses on a surface of the dielectric layer, emittingions through the array of microlenses to implant the ions in thedielectric layer, and removing the array of microlenses from the surfaceof the dielectric layer.

According to another aspect of the present invention, an image sensor isdisclosed. The image sensor includes a pixel layer having aphotodetector portion configured to convert light absorbed by the pixellayer into an electrical signal. The image sensor also includes adielectric layer formed on a surface of the pixel layer. The dielectriclayer has a refractive index that varies along a length of thedielectric layer.

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

What is claimed:
 1. A method for fabricating an image sensor, the imagesensor comprising a pixel layer and a dielectric layer, the methodcomprising the steps of: forming an array of microlenses on a surface ofthe dielectric layer wherein the ions are aligned with photodetectors inthe pixel layer; emitting ions through the array of microlenses toimplant the ions in the dielectric layer; and removing the array ofmicrolenses from the surface of the dielectric layer.
 2. The method ofclaim 1, wherein the forming step comprises: forming an array of polymermicrolenses.
 3. The method of claim 1, wherein the forming stepcomprises: forming the array of microlenses such that points of minimumthickness of the array are centered above respective photodetectorportions of the pixel layer.
 4. The method of claim 1, wherein theforming step comprises: forming the array of microlenses such thatpoints of maximum thickness of the array are centered above respectivephotodetector portions of the pixel layer.
 5. The method of claim 1,wherein the emitting step comprises: emitting the ions in a directionnormal to the surface of the dielectric layer.
 6. The method of claim 1,wherein the emitting step comprises: emitting the ions in a directionoblique to the surface of the dielectric layer.
 7. The method of claim1, wherein the emitting step comprises: emitting ions with an energy ofapproximately 30 keV.
 8. The method of claim 1, wherein the dielectriclayer comprises silicon nitride and the emitting step comprises emittingargon ions.